Autofocusing systems useable in an image acquisition system (hereinafter, an imaging system) can be categorized according to the arrangement of their optical assemblies. That is, a system is known according to whether it focuses through an image-taking optical system or through an independent optical system. Hence, one type of focus-detecting device is known in the art as a through-the-lens (TTL) phase-correlation autofocus (AF) system.
One exemplary construction of the optical system used in a TTL autofocus system is shown in FIG. 1. First and second object images are passed through an objective lens 12 at respective first and second portions distant from the optical axis 13. At a position equivalent to a predetermined focal plane 14 of the objective lens 12, there is disposed an autofocus module which may include a condenser lens 16, a pair of image-forming lenses 18 and 20, and a linear array of photoelectric conversion devices, in the form of line sensor 21 having sections 22 and 24, disposed on the image-forming planes of image-forming lenses 18 and 20. Linear array sections 22 and 24 are respectively composed, for example, of first and second pluralities of photo diode cells a.sub.1 -a.sub.10 . . . , and b.sub.1 -b.sub.16 . . . The output of each cell in array section 22 is provided to a correlation system 28 to be sequentially converted to a digital signal D(X)(X=1,2,3,4,5, . . . ). This image signal data D(X) may then be provided to suitable means (not shown) for performing known AF calculations.
As shown in FIG. 2, in a front-focus condition, an object image of the subject of the focus detection will be formed in a front-focus plane 14A in front of the predetermined focal plane 14B) of the objective lens 12. Two separation images in such a front-focus condition are thereby formed by image-forming lenses 18 and 20 onto the sensor plane 25. Line sensor sections 12 and 14 thus register separation images at positions A, B that are respectively near the optical axis 13. In a rear-focus condition (wherein the object image is formed at plane 14C behind the predetermined focal plane 14B), the two separation images are formed at respective positions E, F at positions remote from the optical axis. In an in-focus condition (wherein the object image is formed on the predetermined focal plane 14B), the two separation images are formed at respective positions C, D on sensor plane 25. (The positions A, B, C, D, E, and F are merely illustrative of their relative locations on the sensor plane and are not meant to be limited to a certain cell.)
Accordingly, as the image patterns of the light distributions on line sensor sections 22 and 24 are converted into respective electrical signals, the relative focus condition can be discovered by AF computing means which process these electrical signals to detect the image position separations AB, CD, EF. Concurrently, the position of the objective lens 12 may be adjusted, until the desired in-focus condition (i.e., image separation CD) is detected.
Conventional through-the-lens correlation autofocus systems are therefore directed to detecting "defocus", which is an indication of the relative absence of focus with respect to a subject. The actual or "absolute" distance of one or more subjects from the autofocus system is not determined. Accordingly, this type of autofocus system has heretofore been unsuitable for supporting camera operations that require such a determination of the absolute distance(s) between the imaging system and the subject(s).
The conventional method for absolute distance determination is based on the triangulation of infared signals that are gathered passively or are produced by an emitter and reflected from the subject to be focussed. In conventional practice, the camera designer is therefore forced to substitute (or add) an active or passive infrared rangefinding autofocus system if the absolute camera-to-subject distance(s) are to be determined. Unfortunately, this sort of design compromise can be undesireable for several reasons, such as increases in camera cost, complexity, and weight.
Furthermore, most passive and all active infrared rangefinding systems cannot successfully measure very long camera-to-subject distances. If such systems are used, the hyperfocal distance of the camera lens must be limited to approximately twice the maximum range of the focussing system, and the focal length and aperture of the camera lens must be limited. In contrast, a through-the-lens correlation autofocus system has no limit on focussing range and therefore frees the designer from this hyperfocal distance requirement.
One very useful camera operation that requires information on absolute subject distance is known as autocomposition, wherein the focal-length of the taking lens (e.g., a zoom lens) is automatically adjusted so as to properly frame the subject(s) in an image frame. This operation typically requires that the subject(s) be determined according to their relative sizes with respect to a predefined categorization of sizes (e.g., landscape vs. portrait modes, etc.) Typically, absolute distance is measured by use of a rangefinding system and the subject category is predetermined by the camera design (e.g., by a camera having a fixed focal length lens), or is acquired upon operator input (e.g., by manipulation of switch means).
Accordingly, there is a need for a through-the-lens autofocus system that is capable of determining absolute distance measurements of the subject-to-camera distance(s), so that related operations such as autocomposition may be effected more simply and easily, and without resort to conventional distance-measuring systems.